1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method which process multivalue image data associated with an area of interest on a print medium to print an image on the area by moving a printing unit relative to the area two or more times or moving a plurality of printing element groups relative to the same area.
2. Description of the Related Art
As a technology to alleviate density unevenness and stripes in an image printed by an inkjet printing apparatus, there has been known a multipass printing method which completes an image on an area of a print medium by performing a plurality of printing scans over that area. Even with the multipass printing method, however, there may occur a shift in a dot printed position between a preceding printing scan and a subsequent printing scan. Such a dot position shift or deviation can lead to variations in dot coverage rate, resulting in image impairments such as density variations and density unevenness.
To alleviate such image impairments, a method has been known which involves dividing image data in the form of multivalue image data before being binarized into different printing scans and then binarizing the divided multivalue image data independently of each other or in an uncorrelated manner (Japanese Patent Laid-Open Nos. 2000-103088 and 2001-150700). FIG. 9A shows an arrangement of dots printed according to image data processed by the method of Japanese Patent Laid-Open No. 2000-103088. In the figure, solid black circles 1501 represent dots printed in a first printing scan; blank circles 1502 represent dots printed in a second printing scan; and gray circles 1503 represent dots printed overlappingly by the first and the second printing scan.
With this arrangement, if a group of dots printed in the first printing scan and a group of dots printed in the second printing scan are shifted in the main scan direction or subscan direction, the dot coverage rate over the print medium does not change so much. The reason for this is that while areas newly emerge in which dots printed in the first scan and dots printed in the second scan overlap each other, there are also areas where two dots that are supposed to overlap each other do not overlap.
However, positively trying to make dots overlap by using the method of Japanese Patent Laid-Open Nos. 2000-103088 and 2001-150700 can deteriorate graininess and insufficient density. For example, in highlighted areas where graininess shows easily, it is preferred that a small number of dots (1701, 1702) be evenly scattered, a predetermined distance apart from each other, as shown in FIG. 9B. However, in the construction disclosed in the Japanese Patent Laid-Open No. 2000-103088, there occur in places areas in which dots are printed overlappingly (1603) or adjoiningly (1601, 1602), as shown in FIG. 9C. These dot lumps show and degrade the graininess. In high-density regions where importance is given to maximum density value, too many overlapping dots can expose blank areas, resulting in an insufficient density. So, the percentage of areas where dots overlap (dot overlap rate) is preferably adjusted not to cause undue density unevenness, graininess and insufficient density in output images.
Particularly when images are formed using a plurality of kinds of ink with different levels of brightness, the density unevenness, graininess and insufficient density vary from one ink to another. That is, to which of them—the restraining of density unevenness, the reduction in graininess and the avoidance of insufficient density—and to what extent the priority should be given changes depending on the kind of ink. So, when two or more colors of ink are used, it is desired that the dot overlap rate be properly adjusted according to the ink color.
In the method disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2001-150700, although descriptions are made of the causes for density unevenness, no considerations are given to a level of density unevenness and a magnitude of increase in graininess resulting from the density unevenness reduction treatment. Therefore, the across-the-board execution of one density unevenness reduction treatment could result in an increased graininess which in turn causes a far worse image degradation than that which could have been caused by the inherent density unevenness. Further, the method disclosed in Japanese Patent Laid-Open Nos. 2000-103088 and 2001-150700 does not consider the fact that the level of density unevenness and the magnitude of an increase in graininess that results when the density unevenness reduction treatment is performed vary from one ink to another. For that reason, there has been a problem that density unevenness and graininess is hardly balanced.